Representing Chemical Changes (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
Representing Chemical Changes
Both, physical and chemical changes can be represented with balanced equations
A balanced equation shows the chemical symbols of all the species and their state of matter
The state symbols are written as subscripts on the right side of each formula
Additional to the three states of matter, there is aqueous state which means dissolved in water
The table below show the four state symbols used in equations
The four state symbols that show the physical state of substances
Solid | Liquid | Gas | Aqueous |
|---|---|---|---|
(s) | (l) | (g) | (aq) |
An equation is divided in reactants (starting substances) and products (substances produced)
In physical changes, the reactants are the same as the products but in different states of matter
In chemical changes, the reactants and the products have totally different identities
Balanced equations representing physical and chemical changes
A physical and a chemical change represented by balanced equations. All the reactants and products show their state symbols
Chemical Equations
Chemical changes are represented by balanced chemical equations
During a chemical reactions, atoms cannot be created or destroyed
This shows that the mass is conserved
An equation is balanced when there is the same number of atoms of each element on both sides of the equation
The numbers in front of chemical formulas are called coefficients
The coefficients show the amount of molecules involved during a chemical reaction
Balanced chemical equation
Balanced chemical equation showing the reaction between hydrogen gas and oxygen gas to produce water. The number of atoms of hydrogen and oxygen is the same in reactants and products
Ionic Equations
When reactions occur in aqueous media, there are three different ways of representing a balanced equation: molecular, complete ionic and net ionic
A molecular equation shows the complete chemical formula of reactants and products
In aqueous solution, the ionic compounds dissociate into their ions. Therefore, they can be written in a separate way
A complete ionic equation shows all the soluble compounds separated into their ions
If an ion appears on both sides of the complete ionic equation, it is called an spectator ion
Spectator ions remain unchanged on each side of the equation
A net ionic equation can be obtained when spectator ions are canceled out front he complete ionic equation
A net ionic equation just includes ions and molecules involved in the reaction
The charge is also conserved in complete ionic equations and net ionic equations. Therefore, the charge must be the same on both sides when the equation is balanced
Three ways to represent a reaction in aqueous solution
Molecular equation, complete ionic equation and net ionic equation for the reaction of lead(II) nitrate and potassium iodide to produce lead(II) iodide and the potassium nitrate
Worked Example
Write down:
A molecular equation
An ionic equation
A net ionic equation
For the following chemical reaction:
zinc + copper(II) sulfate → zinc sulfate + copper
Answer A:
Step 1: Write down the symbol equation showing the reactants and products
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Step 2: Balance the molecular equation an add the state symbols
The equation is already balanced
| Zn | Cu | S | O |
Reactants | 1 | 1 | 1 | 4 |
Products | 1 | 1 | 1 | 4 |
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Answer B:
Step 1: Write down the molecular equation
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Step 2: Break down reactants and products into their respective ions (if available)
Zn (s) + Cu2+ (aq) + SO42- (aq) → Zn2+ (aq) + SO42- (aq) + Cu (s)
Answer C:
Step 1: Write down the complete ionic equation
Zn (s) + Cu2+ (aq) + SO42- (aq) → Zn2+ (aq) + SO42- (aq) + Cu (s)
Step 2: Remove the spectator ions (ions that appear on the left and on the right side of the equation)
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
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